Just now reading the post, M8.
First, I thank you for thinking of me! 
Very interesting design, indeed. I'm wondering about the angle of that fixed square hole and having no adjustment to bring the plate down closer to the fender.
Coincidentally, I have been seriously pondering purchase of a Corbin solo w/back rest. 
If I pull the trigger on a Corbin, I'll have to try and design/fabricate something similar.
I bought the studded Corbin Solo off of our Canuck mate
@Allan S who picked it up with the RIVCO rear rack that sold on Colorado Craigs list or similar site late last year when he was hunting for the rack.
I also got a Utopia backrest like yours Steve and was going to mount it with a cutdown rear pillion seat bracket so I could use it with Solo seats. I have a really old Corbin on my 1978 BMW R100RS outfit and bought a cheap passenger aftermarket adjustable backrest for it with the small square tube. They come with a range of options in size, shape, studs and back plate design. Like this:
US$50 free post.
"NEW" Adjustable / Removable Driver / Passenger Backrest for Corbin Seats | eBay
Just checked and it fits my R3 Corbin Solo too.
I'll have to unzip and remove cover to add studs now to match the Corbin Solo.
The Corbin Solo would not fit easily with my home made studded backrest despite the Roadster Gel Solo fitting with it fine.
The Utopia should fit with the Corbin Solo as it was intended for use with the R3 Pillion seat fitted when required and has a lot mor angle adjustment with the cool utopia mount system, but I haven't tried mine yet.
Why not try a Corbin Solo with your Utopia Steve?
